MORE than 10 billion messages have been sent through Twitter since the microblogging service launched in 2006. The milestone, which was reached Wednesday night, underlines Twitters meteoric growth and increasing popular appeal. In November 2008, it broke the billion tweets mark, and reached five billion tweets last November. However, Twitter users have been unable to identify who sent the 10 billionth tweet, as it came from someone with a protected profile, meaning their messages cannot be viewed by the general public. News that the milestone had been reached came from Gigatweet, the site that keeps a real-time record of all the Twitter messages sent. It is estimated that if Twitter continues growing at its current rate, the 20 billionth tweet will be sent some time in July. The microblogging service is proving increasingly popular among web users, and London is the city with the highest number of Twitter users in the world. Celebrities and sportsmen, including Stephen Fry, Ashton Kutcher and Steven Gerrard are all members of the twitterati, and a growing number of companies are using the platform to promote their brands. But according to a recent survey by Virgin Media Business, only 16 of the UKs biggest 100 listed companies are using Twitter to engage with customers. Around 57 of these FTSE 100 companies had signed up to the service, but 72 per cent had not actively used their account to respond to customer enquiries or comments. Telegraph Twitter recently revealed that users are sending around 600 tweets every second, which equates to more than 50 million tweets per day. One in five tweets now includes a reference to a product, brand or service. Despite its rapid growth, Twitter still falls far behind activity on rival social sites. Facebook users spend eight billion minutes on the site per day, which YouTube serves up a million videos per day. Telegraph